A scissor gear according to the preamble of claim 1 is known from DE102011115692A1. The spring is selected in such a way that the negative torque due to load reversals and crank vibrations are eliminated. The known scissor gear assembly has contact with both the driving flank and coast flank of its mating gear and because of this rattling is prevented. Especially, the rattling noise at low rpm for instance at idling is noticeable and not wanted. The advantage of a flat annular spring compared with a coiled wire compression or coiled wire tension spring is the compact size in axial direction of the scissor gear assembly. This way the body of the scissor gear assembly can be kept thin so it does not need much engine space. The spring load generated by opening or closing the annular spring can be easily transferred from the main gear to the auxiliary gear without creating significant tilting. This because the forces are in planes with relative small offset.
Because of a small interruption in the annular spring forming two spring ends that are close to each other, the spring ends can be positioned in the assembly such way that the spring forces are almost perfectly tangential to the rotation direction of the gear and the spring forces will deliver optimal spring torque. With a small interruption, the spring forces will give no or relative small bending torque in the fixation of the main gear and the fixation of the auxiliary gear. This way the spring fixation can be designed relatively small and the spring section can be designed small towards the ends to have maximum spring stroke.
Using a spring acting in tangential (rotational) direction as an annular spring wherein the spring ends are designed close to each other, no transformation of spring force to tangential direction is required. This in disregard with many scissor gear solutions that make use of an axial or radial acting spring. In the latter two cases the radial or the axial spring force(s), need(s) to be transferred to a tangential (rotational) force to create a torque between the main gear and the auxiliary gear. Such transformation mechanism will cause friction, resulting in efficiency losses and wear.
For small gears and high volume gear projects as usual for automotive engines, the annular spring can be easily produced with fine blanking out of soft material. After the blanking process the annular spring needs to be hardened and tempered to a martensitic microstructure to get the desired spring properties. An alternative heat treatment is austempering to a bainitic microstructure. The latter gives in general smaller heat distortion.
However, for larger annular springs for instance used in truck engines the production volume is relative small. The fine blank tool invest for a big annular spring is becoming relative high. Further the tool gets technically more complex when the size is larger. The distortion of the spring because of the heat treatment will become inacceptable. The tolerance of the distance between the spring ends will give too much tolerance in spring force and the tolerance of the flatness will give assembly problems. This can be solved with high invest and expensive solutions like press hardening and grinding and or machining after the hardening but this will weight on piece price costs and investments.
It has been found that a large annular spring stamped from a relative thin plate (2 to 4 mm) is relative more slender than a small spring made from the same sheet thickness. At a certain moment it is becoming instable because of lateral-torsional buckling. Further, it has been found that at a certain moment the eigenfrequencies are becoming too low and in the range of the engine vibrating frequencies which lead to spring resonance, early breakage of the spring and/or unwanted rattling noise. It has been found that when the spring is placed around the axis of rotation, the rotational speed and torsional vibrations of the shaft where the scissor gear is mounted on, have relative large impact on the resulting spring force. This effect can be positive or negative depending from the application.
Another disadvantage of a single annular spring gear is that the spring will generate a spring force not in line with the gear axis. The main gear and the auxiliary gear rotatable positioned on a mutual axis need a minimum radial play. This to absorb geometrical and dimensional tolerances and to guarantee a minimum oil film. Because of the spring force of the annular spring, the radial play between both gears will be forced to one direction. During rotation of the scissor gear assembly in mesh with another gear, the spring force will continuously change in direction relative to the tooth force of the auxiliary gear in mesh with the other gear. In a certain position during rotation of the scissor gear assembly in mesh with another gear, when the spring force is perpendicular or in the same direction as the radial tooth force, the auxiliary gear will be forced away from the gear in mesh. In another rotation position the spring force will be pointing in a direction opposite to the radial tooth force and will move the auxiliary gear in the direction of the gear in mesh. This continuously changing distance of the auxiliary gear relative to the gear in mesh will give a tooth to tooth transmission error. A higher transmission error generally increases the level of high frequent whine noise (other than low frequent rattling noise).
Further, the annular spring with only one interruption makes it often impossible to provide the main gear with holes for multiple bolts for axial clamping or mounting the gear to an axle or flange. This way applications of such a scissor gear are limited to situations where the fixation to an axle or a flange is with a central bolt or to situations without bolts.
Further, the auxiliary gear should be fixed in axial direction to the main gear in such a way that the fixation is effective, non-expensive and requires no or a little space.